The present invention relates generally to the field of magnetic resonance imaging systems and, more particularly, to the shielding of RF magnetic fields in an open MRI system.
Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. Over the past decades, improved techniques for MRI examinations have been developed that now permit very high quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.
In general, MRI examinations are based on the interactions among a primary magnetic field, a radio frequency (RF) magnetic field and time varying magnetic field gradients with nuclear spins within the subject of interest. The nuclear spins, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of such nuclear spins can be influenced by manipulation of the fields to obtain RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to produce images in MRI systems include a highly uniform, primary magnetic field that is produced by a magnet. A series of gradient fields are produced by a set of three coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. A radio frequency coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spin system from its equilibrium direction, causing the spins to precess around the axis of their equilibrium magnetization. During this precession, radio frequency fields are emitted by the spins and detected by either the same transmitting RF coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to form a final image.
Many specific techniques have been developed to acquire MR images for a variety of applications. One major difference among these techniques is in the way gradient pulses and RF pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios, and resolutions. Graphically, such techniques are illustrated as xe2x80x9cpulse sequencesxe2x80x9d in which the pulses are represented along with temporal relationships among them. In recent years, pulse sequences have been developed which permit extremely rapid acquisition of a large amount of raw data. Such pulse sequences permit significant reduction in the time required to perform the examinations. Time reductions are particularly important for acquiring high resolution images, as well as for suppressing motion effects and reducing the discomfort of patients in the examination process.
In addition to the foregoing developments, new system designs have been developed which allow for greater patient comfort during examination procedures. In one system design, commonly referred to as an opened MRI system, the conventional tubular patient volume defined by the primary magnet and the gradient coils is opened substantially laterally to provide for a greater feeling of space and to reduce patient anxiety. To accommodate the more open patient volume, the conventional tubular magnet and coil structures are separated into a pair of structures which flank the patient volume, typically in upper and lower positions, with a gap provided between the magnet and coil structures. In certain systems, the pair of magnet and coils structures may be provided in a side-by-side arrangement.
Open MRI system design poses a number of technical difficulties both in terms of component design and layout, and in the appropriate control of the system to obtain high-quality image data. For example, because the primary magnet and gradient coils no longer surround the patient in the same manner as in more conventional tubular structures, the entire design of the magnet and coils must be reconsidered to provide the predictable uniform fields needed for encoding the gyromagnetic materials of the patient for image data acquisition, processing and reconstruction. In general, the entire structure of the gradient magnetic field-producing elements is modified to allow for production of the gradient fields and to allow their orientation along desired physical axes for production of images similar to those obtainable on more conventional systems.
The present technique stems from a realization that interactions between the various magnetic fields produced during an examination procedure may adversely affect the performance of the system components as well as the image data obtained. For example, the proximity of an RF magnetic field-producing element and gradient magnetic field-producing elements on either side of the patient volume may lead to field interactions which are undesirable or may lead to degradation in the image data. Specifically, when appropriate pulses are applied to an RF magnetic field-producing element during an examination sequence, RF energy from the RF field element may penetrate the gradient field elements or the core or associated structures of the primary magnet where it is dissipated due to the lossy medium or to field interaction. To maintain a high efficiency of the RF field element, then, an RF shield may be applied between the RF field element and the gradient field elements or primary magnet components. Such shields do serve to prevent or reduce penetration of the RF magnetic field into the gradient field elements and primary magnet components. However, it has been found that the RF energy can still be dissipated in conventional designs.
There is a need, therefore, for an improved RF shielding technique specifically designed for open MRI systems. There is, at present, a particular need for a technique which will allow for enhanced RF shielding without significant redesign of the structures defining the primary magnet and gradient field elements in existing open MRI designs.
The present invention provides a novel technique for RF shielding in open MRI systems designed to respond to such needs. The technique may be employed in newly-designed systems, but may also be retrofitted into existing designs during regular or special maintenance or enhancement procedures. The technique improves upon RF shield designs which could allow for potentially significant loss of RF energy through incomplete or inadequate RF shielding.
In an exemplary implementation of the present technique, an RF shield is provided in one or both field sets of an MRI system, with each field set including a primary magnet element and gradient field-producing elements. The RF shield includes a first member or members which cover the gradient field elements, such as by extending over a face of the elements adjacent to the patient volume. The shield also includes a skirt-like element which extends laterally around the structures, particularly around the primary magnet, its core, and the associated support structures. The elements of the RF shield may be joined to one another by a conductive extensions or fingers to facilitate fabrication. The resulting shield structure therefore forms a shroud or cap which prevents or substantially inhibits loss of the RF energy both through a face of the gradient field element assembly and around a side of the assembly and the primary magnet.